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What causes the physical appearance and health status of identical twins to diverge with age? In this lesson, students learn that the environment can alter the way our genes are expressed, making even identical twins different. After watching a PBS video, A Tale of Two Mice, and reviewing data presented in the Environmental Health Perspectives article, “Maternal Genistein Alters Coat Color and Protects Avy Mouse Offspring from Obesity by Modifying the Fetal Epigenome,” students learn about epigenetics and its role in regulating gene expression.

Learning outcomes

The learner will:

define the make-up of chromatin.

define the term “epigenetics.”

describe DNA methylation as a mechanism for inhibiting gene transcription.

describe how gene expression can vary among genetically identical offspring.

Teacher planning

Time required

45-60 minutes
Depending on student proficiency level, this lesson can be completed as a homework assignment to encourage independent student work and/or to save class time.

Materials needed

Student worksheet, one copy per student

Technology resources

Computer with Internet access and audio (sound) capabilities

LCD Projector

DNA wrap slide library

Open as PowerPoint (1 MB)

Student handouts

Environmental Health Perspectives Epigenetics worksheet

Open as PDF (523 KB, 3 pages)

Environmental Health Perspectives Epigenetics worksheet answer key

Open as PDF (596 KB, 3 pages)

Pre-activities

Prior knowledge and vocabulary

This lesson is best conducted after students have learned about general DNA structure and function, transcription and translation, and general regulation of gene expression.

Ensure students have a basic understanding of DNA structure and function prior to introducing the concept of epigenetics. Students should already be familiar with the following terminology:

Chromatin

Chromosome

Deoxyribonucleic acid (DNA)

Gene

Gene expression/regulation

Histone proteins

Nitrogenous base

Nucleoprotein

Nucleotides

Phosphodiester bond

Promoter

Ribonucleic acid (RNA)

Transcription

Transcription factors

Translation

Teacher preparation

Review the supplemental information and instructions for this activity. Additional resources are listed in the Resources section.

Distribute copies of the Student Worksheet and ask students to work in pairs to complete Step 1.

Promote a brief class discussion by asking students to share their answers to Step 1, questions 1 and 2, with the class.

What does it mean to say that two individuals are genetically identical?

How can two genetically identical mice look so different?

Tell students that the answer to these questions lies in how DNA is packaged inside cells and then invite students to complete Step 2 on their worksheet. The amount of time students need for this step depends on the extent to which DNA and chromosome structure has already been covered in class.

Review student answers to Step 2 before proceeding to Step 3.

Next, introduce students to the concept that changes in gene expression can occur without changes in the DNA sequence of genes (mutation). Describe the process of DNA methylation as a means of silencing transcription of a gene (as described in the Background section).

To conclude this description of DNA methylation, return to the audio slide show A Tale of Two Mice. and show students Chapter 2, “The Epigenome” (1:06), and at least the first 15 seconds of the next chapter, “Switching on the Agouti Gene,” which explains that in the yellow, obese mouse, the agouti gene is unmetlyated and turned on all of the time while in the brown mouse the gene is completely methylated and shut down.

Ask students to complete Step 3 by summarizing in their own words how DNA methylation affects DNA structure and function. Review student responses as a class before proceeding.

Next, ask students to read Step 4, examine Figures 4 and 5 from the featured Environmental Health Perspectives article and answer the corresponding questions. Optional: Expose students to the original article and have them read and discuss all or part of this scientific publication.

Review student responses as a class before allowing them to proceed to Step 5.

Ask students to read Step 5, and with a partner, discuss how the authors’ conclusions are significant to them.

Assessment

Ask students to turn in their completed worksheets (Answer Key provided).

Ask student to summarize, in their words, what they learned during this activity.

Ask students to construct a concept map using critical vocabulary terms (see below) along with vocabulary terms from page 2 to demonstrate they understand the concept of epigenetics in the context of DNA structure and function.

Supplemental information

Deoxyribonucleic acid (DNA) is a large, complex molecule (macromolecule) that contains the genetic code or the information needed to direct the activities of a cell and for transmission of this information to the next generation. A single DNA strand is made up of building blocks called nucleotides that are connected together like a chain. Each DNA nucleotide is composed of a nitrogenous base—either adenosine (A), guanine (G), thymine (T), or cytosine (C)—a five-carbon deoxyribose sugar (S), and a phosphate group (P). A gene is a specific sequence of nucleotides within a DNA strand that provides the instructions necessary to carry out a particular activity.

DNA exists as a double-stranded polymer of nucleotides that forms a helix in which two DNA strands run anti-parallel to one another and interact via hydrogen bonds between the nitrogenous bases. The hydrogen bonds between the nitrogenous bases can be broken to allow the DNA strands to separate during DNA replication and gene expression, which occurs when the nucleotide sequence of a gene is copied into ribonucleic acid (RNA) during a process called transcription. For genes that encode proteins, DNA is copied into messenger RNA (mRNA), which then directs the synthesis of proteins during the process known as translation. Gene expression is highly regulated in order to control a cell’s activities; thus, the timing and amount of RNA and protein generated from a given gene varies depending on the cell’s activities. Disruption of gene expression regulation leads to diseases such as cancer.

Inside cells, DNA is packaged around proteins called histones; this DNA–protein (nucleoprotein) complex is called chromatin. Histones act like “spools” around which DNA is wrapped. In humans, each cell contains approximately 2 meters of DNA; however, because of the wrapping of DNA around histones, the condensed DNA is approximately 120 micrometers long!

This DNA “packaging” in the form of chromatin plays a key role in the regulation of gene expression. The nucleoprotein inside cells serves as a docking site for the different proteins and enzymes and their interactions required for DNA replication, transcription, recombination, and repair.

This lesson introduces students to the emerging field of epigenetics. Epigenetics literally means “on top of or in addition to genetics,” and is the study of changes in gene expression not accompanied by alterations in DNA sequence. In parallel to the term genome, which defines the complete set of genetic information contained in the DNA of an organism, epigenome refers to the complete set of epigenetic pathways in an organism. Epigenetic modifications to DNA exert profound influences on gene activity. For example, studies suggest that epigenetic variation may be responsible for subtle differences in appearance and behavior of identical twins. Identical twins are more epigenetically similar early in life but show remarkable divergence with age.

Epigenetic pathways such as DNA methylation and histone modifications interact with each other to regulate expression of genes. One of the most common and well-characterized epigenetic pathways is DNA methylation. DNA methylation occurs when an enzyme called a methyltransferase covalently attaches a methyl (-CH3) group to a cytosine base that is adjacent to a guanine base (see Figure 1). Such sites where a cytosine is adjacent to guanine via a phosphodiester bond are called CpG sites. Scientists have observed that DNA methylation occurs predominately along places on the DNA strand that are rich in CpG pairs. One type of CpG-rich region is a CpG island. CpG islands are associated with approximately 60-70% of mammalian genes, and most CpG islands are unmethylated in normal mammalian cells. Thus, changes in methylation patterns at CpG islands can interfere with normal gene expression by altering the transcriptional competency of a gene’s promoter. Genes that are essential for a cell’s function are not methylated. In contrast, inactive genes are usually methylated to suppress their expression.

While DNA methylation is involved in normal control of gene expression, changes in the extent of DNA methylation can contribute to cancer or disease by silencing genes that that should otherwise be active or expressed or by causing expression of genes that are usually inactive. Methylation is one mechanism for suppressing (or silencing) gene transcription by preventing one or more transcription factors (TF) and thus RNA polymerase from accessing a gene’s promoter which is required for transcribing DNA into RNA (see Figure 2).

The histone proteins that hold DNA tightly wound inside each cell can also be modified by methylation or other modifications such as acetylation or phosphorylation. When too much or too little of a given histone modification occurs, it affects a gene’s expression and consequently its function, which causes unwanted alterations in the cell, potentially resulting in disease.

Figure 2: The addition of methyl groups to CpG islands common to promoters is one mechanism for suppressing (or silencing) gene transcription (Image: Fry, 2011).

Epigenetic modifications can be maintained and inherited by daughter cells during mitosis and to a lesser extent during meiosis. Therefore, epigenetic modifications that occur in utero can be passed on to subsequent generations. Environmental factors such as exposure to heavy metals (arsenic, nickel) and cigarette smoke, and dietary factors such as vitamin and folate deficiencies have been linked to abnormal changes in epigenetic pathways, suggesting that an individual’s environment plays an important role in shaping their epigenome. Epigenetic changes have been observed in different stages of cancer progression, in the process of aging, and in other human diseases such as Alzheimer’s disease, diabetes and obesity.

In A Tale of Two Mice, the narrator discusses the difference in coat color between two genetically identical mice. The obese, yellow mouse has an unmethylated Agouti gene, which is constantly being expressed (when it normally should be “off ” or silenced), while her sister, the brown mouse, has a methylated Agouti gene that has permanently been turned “off ” and thus is not expressed. Although genetically identical in terms of the DNA sequences they’ve inherited from their mother and father (the mice are inbred), epigenetic modifications have led one mouse to be overweight and more susceptible to diabetes and cancer. This difference in gene expression between the genetically identical mice can be attributed to differential gene expression as a result of epigenetic modifications.

Critical vocabulary

refers to heritable changes in the regulation of the expression of gene activity without alteration of genetic structure

epigenetic modifications

chemical compounds (e.g. methyl groups) that are added to single genes can regulate their activity; these modifications are known as epigenetic changes. These chemical “tags” can remain in place as cells divide and in some cases can be inherited through generations

epigenome

refers to all of the chemical compounds that have been added to the entirety of one’s DNA (genome) as a way to regulate the activity (expression) of all the genes within the genome. Environmental influences, such as a person’s diet and exposure to pollutants, can also impact the epigenome

methylation

small molecules called methyl groups, each consisting of one carbon atom and three hydrogen atoms, are covalently attached to segments of DNA by an enzyme known as a methyltransferase. When methyl groups are added to a particular gene, that gene is turned off or silenced, and no protein is produced from that gene.

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